1. Field of the Invention
The present invention relates to an interferometer-type radar which observes a flying target.
2. Description of Related Art
Conventionally, an FMCW (Frequency Modulated Continuous Wave) radar has been used as a means for knowing the distance to a target and the velocity of the target, other than a pulse radar. In a conventional FMCW radar, a receiving signal is frequency-converted with the same FMCW signal as a transmitting signal, whereby a baseband signal is generated. The velocity of the target is detected from the Doppler shift component of the baseband signal frequency in the frequency ascending time and the frequency descending time of the FMCW signal, and the distance to the target is obtained from the baseband signal frequency by correcting the Doppler component (see, for example, Japanese Patent Application Laid-open No. 2000-46941 and Japanese Patent Application Laid-open No. 2003-177175). Further, the present inventor also proposes an interferometer-type radar capable of measuring distance and direction with high sensitivity by applying the FMCW radar technique.
However, when the target is moving at a high speed, there is a problem that a baseband signal, which is a frequency-converted receiving signal by a transmitting signal, is affected by a Doppler shift due to the velocity of the target, and at the same time, the baseband frequency changes according to the movement of the target, whereby it is impossible to measure the distance accurately. Further, if the velocity and a change in the velocity of the target are large, or if there are a number of targets of different velocities at the same time, it is difficult to apply the above-mentioned method.
In view of the above, the present inventor developed an interferometer-type radar in which problems of a conventional FMCW radar were solved. In order to realize a highly sensitive interferometer-type radar, it is required to increase the number of elements of a receiving antenna, and to integrate correlation-processed signals for a long time.
However, if the number of elements of the receiving antenna increases, alignment of the antenna is extended so the total process gain obtained by interference processing increases, but at the same time, the directivity of correlation processing becomes sharp. When a target moves at a high speed, the output phase of the interferometer changes according to the movement of the target. Thereby, the integration efficiency is lowered or an integration time cannot be secured, causing a problem that the sensitivity deteriorates comprehensively.
Further, in the case where the distance of a target moving at a high speed is observed by an FMCW radar with high accuracy, a reflecting signal from the target has a Doppler shift, and the distance change within the sweep cycle cannot be disregarded, so it is difficult to obtain the position of the target with high accuracy.